Transdermal drug delivery is an option in situations where other means of therapy have failed
Transdermal drug delivery is an option in situations where other means of therapy have failed. Drugs with an easily measured endpoint and a wide therapeutic window should be used to provide maximum safety. Care should be taken that the condition being treated is not serious, such as a major infection being treated with transdermal antibiotics. It may take trial-and-error to establish an effective medication and dose for a particular patient and some margin of safety should be provided.
Marks SL, Taboada J: Transdermal Therapeutics. J Am Anim Hosp Assoc 39:19-21, 2003.
The unpalatable nature of medications coupled with difficulty in swallowing oral products and poor owner compliance often make delivering medication to animals a challenge. Transdermal therapy can avoid these problems since the skin, the largest organ of the body, is easily accessible by the owner for medication delivery. Transdermal medications can be administered to any hairless part of an animal such as the inside of the ear or a shaved part of the animal's body.
Advantages of transdermal administration include increased owner compliance, avoidance of first-pass hepatic metabolism, and ease of administration. Disadvantages include variability in absorption, local reactions to the drug or transdermal vehicle, lag time between application and systemic absorption, and difficulty reversing toxicity of a drug residing in the skin. Drugs mentioned in the article that are being compounded for transdermal use include aminophylline, amitriptyline, buspirone, chloramphenicol, cisapride, diclofenac, diltiazem, diphenhydramine, enalapril, enrofloxacin, famotidine, furosemide, hydroxyzine, ibuprofen, itraconazole, ivermectin, ketoprofen, lorazepam, methimazole, metronidazole, prednisone, and urosdiol. Despite the advantages of transdermal therapy in veterinary patients, there is little evidence in the literature to support the use of these drugs.
The small animal epidermis has five main layers: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and the stratum basale. The stratum corneum appears to be the rate-limiting point for transdermal absorption. Transdermal drug absorption occurs primarily by passive diffusion through the skin. Concentration of the drug, surface area of applied drug, thickness of the skin, and diffusion and partition coefficients of the drug affect this interaction. Other factors, such as shaving, hydration, and body temperature, affect the permeability of the stratum corneum. Absorption can vary between species, animals within a species, and in an individual animal.
Transdermal delivery can be a useful option, however, several factors must be taken into account to ensure efficacy. Suitability of the drug molecule to pass through the skin is an important consideration for transdermal drug delivery. Compounds must be lipophilic, low molecular weight, (<400 Daltons), and have a high melting point and partition coefficient. Drug stability is an issue as the drug may be combined with a variety of different carriers and excipients. A common carrier for transdermal drug delivery, PLO (pluronic lecithin organogel), promotes the formation of drug micelles that pass through the skin. Compounding pharmacies are formulating more and more medications for transdermal use, but not all drugs may be suited for transdermal delivery. The few controlled studies involving transdermal use in veterinary patients provide little information as to which drug applications are actually effective. For the most part, drugs that have a defined therapeutic endpoint or laboratory monitoring capability are the most suitable for transdermal use. Use of these formulations should be at the discretion of the provider and individualized for each patient.
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